Because you can't get to the centering ball to grease that unless you remove the shaft.

 

Is that a particular issue on these, or is that just the philosophy of thoroughness? I've always pulled a used shaft the first time I service a truck and lube that joint thoroughly and treat it as a lifetime lube. But then, I've never subjected any of my trucks to particularly harsh conditions either.

 

Probably more just thoroughness. Only takes about 15 mins to remove and 15 mins to put back. Besides lubrication gives you a good change to feel the wear in the joints. If it lets loose while driving you will have a very bad day.